The present invention generally relates to the calibration of devices used for measuring electrical power and energy.
Power in an electric circuit is the rate of flow of energy past a given point of the circuit. In alternating current (AC) circuits and loads, energy storage elements such as inductors and capacitors may result in periodic reversals of the direction of energy flow. The portion of power that, when averaged over a complete cycle of the AC waveform, results in net transfer of energy in one direction is known as real or active power. The portion of power due to stored energy, which returns to the source in each cycle, is known as reactive power.
Electricity meters operate by continuously measuring the instantaneous voltage and current to give energy used in joules or kilowatt-hours. Meters for smaller services, such as small residential customers, can be connected directly in-line between source and customer. For larger loads, more than about 200 amperes, current transformers are used so that the meter can be located other than in line with the service conductors. Meters fall into two basic categories, electromechanical and electronic.
The most common type of electricity meter is the electromechanical induction watt-hour meter which operates by counting the revolutions of a non-magnetic, but electrically conductive, metal disc which is made to rotate at a speed proportional to the power passing through the meter. However, electronic electricity meters are increasingly being installed as they offer many advantages. Electronic meters display the energy used on an LCD or LED display, and some can also transmit readings to remote places. In addition to measuring energy used, electronic meters can also record other parameters of the load and supply such as instantaneous and maximum rate of usage demands, voltages, power factor and reactive power used, etc.
Conventional meters include electronic components and circuits which can introduce gain errors and phase delays into the measurement process. These errors and delays can vary between manufactured units. They can also vary across the gain range of the meter and with the phase angle between the voltage and current at the meter. It is these errors and delays which necessitate calibration of the meter.
Conventional calibration techniques can involve many measurements including active power measurements, reactive power measurements, measurements at different gain levels, measurements at different phase angles, etc. Conventionally, each of these measurements is performed manually.
A conventional meter calibration set-up will be described using a block diagram.
FIG. 1 shows a block diagram 100 illustrating a conventional meter calibration system.
Block diagram 100 includes a power source tester 102 and a meter 104.
Power source tester 102 is arranged to connect to meter 104 via a power line 108. Meter 104 also connects to power source tester 102 via a line 106.
Power source tester 102 is operable to provide an accurate test voltage, current and phase angle to meter 104. Meter 104 is operable to measure power and to generate calibration pulses 114 to power source tester 102 via line 106.
A voltage 110 and a current 112 represent the voltage and current components, respectively, of power line 108. In operation, voltage 110 and a current 112 represent an accurate reference power with a settable voltage to current phase angle. These are configured and generated at power source tester 102 for the purpose of calibration of meter 104. Meter 104 measures the reference power provided, accumulates power measurements over time and generates a calibration pulse when it has reached a certain predetermined energy threshold. Power source tester 102 then compares the reference energy and measured energy to determine any error. Meter 104 is then manually adjusted to minimize the error.
In practice, multiple measurements are made at different gain settings across the gain range of meter 104. The gain under test is determined by the current setting at power source tester 102. Active power measurements are made when voltage 110 and current 112 are generated in phase with each other. For reactive power measurements, the required phase angle between voltage 110 and current 112 is set at power source tester 102.
Electrical meter calibration, therefore, is conventionally a time-consuming, labor intensive process with multiple test stages being run and multiple reference currents, voltages and phase angles required to be set up at the calibration equipment before tests are run. After testing, manual adjustments need to be made to the meter units under calibration in order to compensate for errors.
What is needed is a system and method that can automate the calibration of electric energy meters and minimize the external test equipment necessary to perform the calibration.